Apparatus for measuring the temporal correlation of fundamental particles

ABSTRACT

An apparatus for measuring the temporal correlation of fundamental particles such as photons, neutrons, X-rays or the like, comprising at least one deflector for sweeping the fundamental particles or images thereof in at least one direction, an aperture member having at least two apertures for time-divisionally extracting the swept fundamental particles or the images thereof, multiplication means, such as dynode groups or photomultipliers, for multiplying each of the extracted fundamental particles or the images thereof and a correlator for performing correlating arithmetic operations on the basis of each output signal from the multiplication means.

This is a continuation of application Ser. No. 07/246,106 filed Sep. 19,1988, the contents of which are herein incorporated by reference, andissued as U.S. Pat. No. 4,967,080 on Oct. 30, 1990.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for measuring the temporalcorrelation of photons, neutrons and other fundamental particles.

There has been known an apparatus for measuring the temporal correlationof photons detected from incident light.

The operating principle of the apparatus for measuring photon's temporalcorrelation is shown in FIG. 1. A photon beam is divided by a beamsplitter 71 and the resulting two beams are directed into detectors 72and 73. By changing the time-dependent parameter for each detector, thetemporal correlation of photons can be measured with a correlator 74.Each of the detectors 72 and 73 comprises a photomultiplier tube, asemiconductor detector or the like.

If the respective distances from the beam splitter 71 to the detectors72 and 73 are l₁ and l₂, the difference in arival time τ (the differencebetween the times at which photons arrive at the detectors 72 and 73) isexpressed as:

    τ=(l.sub.1 -l.sub.2)/c                                 (1)

where c is the velocity of light. By measuring the probability thatphotons are simultaneously detected with detectors 72 and 73, thetemporal correlation of photons within a time interval of τ can bedetermined.

FIG. 2 shows schematically a conventional apparatus for measuringtemporal correlation using a single detector, or a photomultiplier tube80. A photon passing through light attenuating filters 82 and a pinhole81 is detected with the detector 80, for example a photomultiplier tubeand the signal line after the detection is divided into two lines, oneof which is delayed for determining the temporal correlation of photonwith a correlator 83.

With the conventional temporal correlation measuring an apparatus asdescribed above, photons to be measured are admitted into the detectors72 and 73 or detector 80 in a direct way, namely, sequentially on a timebasis. In order to obtain temporal correlation for a very short timedifference of the order of subnanoseconds with this mechanicalarrangement, not only the detectors 72 and 73 or detector 80 but alsothe correlator 74 or 83 is required to have high-speed responsecharacteristics for providing a high time-resolving capability. However,in the the conventional technology, since the detectors 72 and 73 ordetector 80 comprises a photomultiplier tube or semiconductor detector,the response speed thereof cannot be made faster than 10 picoseconds.

Further there also is a limit on the efforts that can be made to realizehigh-speed response with the circuits constituting the correlator 74 or83.

As described above, the conventional temporal correlation measuringapparatus has a limited capability for determining temporal correlationfor a very short time difference with high temporal resolution.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus that iscapable of measuring the temporal correlation of fundamental particlesfor short time difference of the order of subpicosecond with hightemporal resolution.

Another object of the present invention is to provide an apparatus thatis capable of measuring the temporal correlation of fundamentalparticles using a simplified processing system.

A further object of the present invention is to provide an apparatusthat is capable of successively measuring the temporal correlation offundamental particles while varying time differences.

The above objects are obtained by the provision of an apparatusaccording to this invention comprising: sweep means for sweepingfundamental particles or images thereof, extracting means fortime-divisionally extracting the swept fundamental particles or imagesthereof through plural apertures formed in the extracting means,multiplication means for multiplying each of the fundamental particlesor images thereof extracted through the apertures, and correlation meansfor performing correlating arithmetic operations on the basis of eachoutput signal from the multiplication means.

The sweep means in the apparatus of the present invention sweepsfundamental particles such as photons, neutrons or the like or imagesthereof, for example, in the horizontal or vertical direction, or in theboth directions. The swept particles or images thereof are extractedthrough plural apertures within a time difference that is determined bythe sweep speed and the distance between apertures. The extractedfundamental particles or images thereof are multiplied by themultiplication means. The temporal correlation of fundamental particlesis measured with the correlation means on the basis of each outputsignal from the multiplication means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show schematically conventional apparatuses for measuringtemporal correlation of fundamental particles;

FIG. 3 shows schematically an apparatus for measuring temporalcorrelation of fundamental particles in accordance with a firstembodiment of the present invention;

FIG. 4 shows schematically an aperture member as shown in FIG. 3;

FIG. 5 shows a second embodiment of the apparatus as shown in FIG. 3;

FIG. 6 shows a third embodiment of the apparatus as shown in FIG. 3;

FIG. 7 shows schematically another aperture member employed in theapparatus of the third embodiment;

FIG. 8 shows a fourth embodiment of the apparatus as shown in FIG. 3;

FIG. 9 shows a fifth embodiment of the present invention;

FIG. 10 shows schematically the aperture member as used in the apparatusas shown in FIG. 9;

FIG. 11 is a graph showing the results of correlation measurement;

FIG. 12 shows a sixth embodiment of the present invention; and

FIGS. 13 and 14 show seventh and eighth embodiments of the apparatus ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention are described hereinafterwith reference to the accompanying drawings.

FIG. 3 is a schematic diagram for showing an apparatus for measuring thetemporal correlation of fundamental particles according to a firstembodiment of the present invention.

The apparatus as shown in FIG. 3 comprises a member 11 having anaperture 10 for guiding fundamental particles such as photons, neutrons,X-rays or the like, a fundamental particle-to-electron convertingsurface 12 such as a photocathode for receiving the fundamentalparticles that have passed through the aperture 10 and a focusing lenssystem 50, an accelerating electrode 13 for accelerating the electronimage of fundamental particles from the surface 12, a focusing electrode14 for focusing the accelerated electron image of fundamental particles,a deflector 15 for sweeping the electron image of fundamental particles,an aperture member 19 for extracting the swept electron image offundamental particles through two apertures 17 and 18, two dynode groups20 and 21 for respectively producing multiplied outputs of the electronimages of fundamental particles extracted through the two apertures 17and 18, and a correlator 22 for performing correlating arithmeticoperations on the basis of each output signal from the two dynode groups20 and 21.

The deflector 15 is designed to generate an electric field in responseto an applied voltage, and comprises, for example, a pair of deflectionplates.

The aperture member 19 has a structure as shown in FIG. 4, in which twoapertures 17 and 18 are formed as parallel slits that are elongated in agiven direction, for example, in a horizontal direction.

The correlator 22 comprises amplifiers 23 and 24 for amplifying theoutput signals from two dynode groups 20 and 21, discriminating/shapingcircuits 25 and 26 for performing peak discrimination and waveformshaping of the output signals from the amplifiers 23 and 24,respectively, an AND circuit 27 for subjecting the two discriminated andshaped signals to an AND operation for determining correlation thereof,and a counter 28 for counting and storing the results of correlation assupplied from the AND circuit 27.

The operation of the apparatus having the construction as describedabove will proceed as follows. If fundamental particles such as photonspass through the aperture 10 and an image thereof is formed on thesurface 12 by means of the focusing lens system 50, electrons will beemitted from the surface 12 and are admitted into the deflector 15through the accelerating electrode 13 and the focusing electrode 14. Theelectrons are swept by the deflector 15 in a vertical direction asindicated by arrow A in FIG. 4, and are fed into the correlator 22 afterhaving been amplified through two dynode groups 20 and 21 with a timedifference proportional to the distance y (mm) between the two apertures17 and 18 in the aperture member 19. If the deflector 15 is assumed toperform a sweeping operation at a rate of Vs (mm/psec) as measured onthe aperture member 19, the time difference τ, or the difference betweenthe times at which electrons pass through two apertures 17 and 18, willbe expressed as:

    τ=y/Vs                                                 (2)

Then, the correlator 22 is capable of obtaining temporal correlation fora time difference τ (in piscoseconds).

The AND circuit 27 in the correlator 22 produces a high-level outputonly when it is simultaneously supplied with the output signals from thediscriminating/shaping circuits 25 and 26, and the so producedhigh-level outputs are successively counted by the counter 28 to producea desired temporal correlation.

As described above, the apparatus as shown in FIG. 3 converts successiveincident particles into their respective electron images, sweeps theelectrons vertically with the deflector 15, and time-divisionallyextracts the swept electrons through the two apertures 17 and 18 formeasuring the temporal correlation between extracted electrons. Becauseof this arrangement, the dynode groups 20 and 21 and the correlator 22need not have a particularly high response speed to realize highlyefficient time-resolved correlation for a very short time difference.The time difference necessary to attain temporal correlation isdetermined by the sweep speed of the deflector 15 and the distancebetween two apertures 17 and 18, but it is not dependent on the temporalresolution of the processing system behind the aperture plate 19, whichcomprises the dynode groups 20 and 21 and the correlator 22. Therefore,temporal correlation for a very short time difference of the order ofsubpicoseconds can be measured with high temporal resolution. In thisconnection, it should be mentioned that temporal correlation for varyingtime differences can be obtained by changing the sweep speed of thedeflector 15.

A further advantage of the apparatus of the above embodiment of thepresent invention is that the construction of the processing system canbe simplified since one only needs to determine the temporal correlationof the output signals from the two apertures 17 and 18.

FIG. 5 shows a modification of the apparatus as shown in FIG. 3. Theapparatus as shown in FIG. 5 has an electronic lens system 35 betweenthe deflector 15 and the aperture member 19. If the electronic lenssystem 35 is of an enlarging type, temporal correlation for varying timedifferences can be obtained by changing the magnifying power of the lenssystem instead of changing the sweep rate of the deflector 15. If theenlarging electronic lens system is to be attached to an imageintensifier, it may be disposed in front of the latter. Alternatively,it may be adapted to serve both as an electronic lens system and as animage intensifier. If the electronic lens system 35 is designed as anelectron image rotating type, the direction in which electrons are sweptcan be altered. An example of this change in electron sweeping directionis shown in FIG. 4, in which the sweep direction indicated by arrow Awhich is peculiar to the deflector 15 is rotated to the directionindicated by arrow B. As a result of this change in sweep direction, theeffective distance between apertures 17 and 18 is sufficiently changedto measure temporal correlation while varying the time difference. Inthis way, a desired temporal correlation can be obtained in theapparatus as shown in FIG. 5 through proper control of the magnificationand direction of sweeping.

The purpose of performing the temporal correlation measurement withvariable time differences, can also be attained by the design as shownin FIGS. 6 and 7 instead of using the electronic lens system 35. Asshown in FIG. 6, incident light is divided into two beams by a beamsplitter 40. One of the resulting beams passes through an aperture 43and a focusing lens system 51, from which it is directly guided to thefundamental particle-to-electron converting surface 12. The other beamis delayed in time through a delay means 41 and a mirror 42 andthereafter passes through an aperture 44 and a focusing lens system 52,from which it is guided to the surface 12. Electrons are emitted fromthe surface 12 at the positions where it has been struck with theincident fundamental particles such as photons and they pass through theaccelerating electrode 13 and the focusing electrode 14 to be swept bythe deflector 15. The swept electrons are time-divisionally extractedthrough two apertures 45 and 46 in an aperture member 47. The twoextracted electrons are multiplied with dynode groups 48 and 49,respectively, and the output from each dynode group is supplied into thecorrelator 22 for determining temporal correlation. The aperture member47 has apertures as shown in FIG. 7 and electrons that have been sweptby the deflector 15 in the direction indicated by arrow C are extractedthrough the apertures 45 and 46.

As described above, the apparatus as shown in FIGS. 6 and 7 controlstime difference with the delay means 41 so as to obtain temporalcorrelation of fundamental particles as photons, while varying timedifferences.

With reference to FIG. 3, the dynode groups 20 and 21 serving asmultiplication means are accommodated in a single tube together with thedeflector 15 and the aperture member 19. An alternative to thisarrangement is shown in FIG. 8, in which a phosphor screen 36 isdisposed behind an aperture member 19 and photomultiplier tubes 37 and38 are disposed behind the phosphor screen 36 at positions that areassociated with the apertures 17 and 18, respectively. In thisarrangement, electrons that have passed through the two apertures 17 and18 are incident to the phosphor screen 36 and photons that are generatedfrom the screen are supplied into photomultiplier tubes 37 and 38, whichproduce output signals to be fed into the correlator 22.

The apertures 17 and 18 as shown in FIG. 4 are in the form of parallelslits that are elongated in a horizontal direction. If desired, theseapertures may be arranged in such a manner that they are elongated at acertain angle with respect to the horizontal direction.

The foregoing embodiments assumes that the deflector 15 sweeps electronsin a vertical direction but this is not the only way of an electronsweeping operation, and electrons may be swept in any direction so longas they can traverse the two apertures 17 and 18.

FIG. 9 is a schematic diagram for showing the apparatus according to afifth embodiment of the invention.

The apparatus as shown in FIG. 9 has the basically same construction asthat of the apparatus as shown in FIG. 3, except for some componentsthereof. Accordingly, the components which correspond to those as shownin FIG. 3 are identified by like numerals, and will not be described indetail.

In addition to the components as shown in FIG. 3, the apparatus of thefifth embodiment is provided with another deflector 16 for sweeping theelectron images of fundamental particles in the direction different fromthat of the deflector 15, for example, in the horizontal direction, andan aperture member 19' having apertures 17' and 18' as shown in FIG. 10.The deflector 16 is also designed to generate an electric field inresponse to an applied voltage, and comprises, for example, a pair ofdeflection plates. The apertures 17' and 18' are formed in the aperturemember 19' in such a manner that the distance y(x) (mm) therebetweenincreases progressively as the value (x) of a position in a horizontaldirection increases.

The operation of the apparatus having the construction as describedabove will proceed as follows. If fundamental particles such as photonspass through the aperture 10 and an image thereof is formed on thesurface 12 by means of the focusing lens system 50, electrons will beemitted from the surface 12 and are incident on the aperture member 19through the accelerating electrode 13, the focusing electrode 14 and thedeflectors 15 and 16. If the deflector 16 is initialized in such amanner that an electron will first encounter the aperture member 19' ata horizontal position x="0", the electron that has been swept verticallyby the deflector 15 will pass through the two apertures 17' and 18' atposition y(0) (mm) where the distance between these two apertures is thesmallest. If the voltage applied to the deflector 16 is graduallychanged so as to shift successively the horizontal position x whereelectrons are incident on the aperture member 19', the electron sweptvertically by the deflector 15 will each time be supplied to thecorrelator 22 through two dynode groups 20 and 21 with a time differenceproportional to the distance y(x) (mm) between the two apertures 17' and18' in the aperture member 19'. If the deflector 15 is assumed toperform a sweeping operation at a rage of Vs (mm/psec) as measured onthe aperture member 19', the time difference τ, or the differencebetween the times at which electrons pass through two apertures 17' and18', will be expressed as:

    τ=y(x)/Vs                                              (3)

Therefore, by successively shifting the horizontal position x with thedeflector 16, temporal correlations can be successively obtained withthe correlator 22 while varying values of time difference τ.

The AND circuit 27 in the correlator 22 produces a high-level outputonly when it is simultaneously supplied with the output signals from thediscriminating/shaping circuits 25 and 26, and the high-level outputsthus produced are successively counted by the counter 28 to produce adesired temporal correlation. High operational efficiency can beachieved by employing a multi-channel counter as counter 28, which isscanned in synchronism with the horizontally-deflecting operation of thedeflector 16 for shifting the horizontal position x of electrons. Theresults of correlation stored in this way in the multichannel counterare shown in FIG. 11, where each channel corresponds to time differenceτ.

If electrons are swept horizontally by the deflector 16 with thedistance y(x) (mm) between electron extracting apertures 17' and 18'being successively changed, temporal correlations for a broad spectrumof a short time-differences ranging from the order of subpicoseconds toa much longer time-difference can be continuously measured with hightemporal resolution, in addition to the performances obtained by theapparatus as shown in FIG. 3.

FIG. 12 is a schematic diagram as showing an apparatus for measuring thetemporal correlation of fundamental particles according to a sixthembodiment of the present invention. In FIG. 12, the components whichcorrespond to those as shown in FIG. 9 are identified by like numeralsand will not be described in detail.

The apparatus according to the sixth embodiment of the present inventionincludes a correlator 30 in which the output signal that has beenmultiplied by one dynode group, for example, the dynode group 21, issupplied to a variable delay 31 through amplifier 24 anddiscriminating/shaping circuit 26, and the signal supplied to the delaycircuit 31 is further delayed for a certain time before it is fed intothe AND circuit 27. The variable delay circuit 31 may be composed of ashift register which further delays the output signal from thediscriminating/shaping circuit 26 by an amount equal to the product ofthe sweep period of deflector 15 as determined by the signal from asignal generator 32 and the number of count settings.

If the sweep period of the deflector 15 is 10 nanoseconds and the numberof count settings on the variable delay circuit 31 is "3", thearrangement as shown in FIG. 12 will cause the output signal from thediscriminating/shaping circuit 26 to be further delayed for 30nanoseconds. In other words, the two output signals from thediscriminating/shaping circuits 25 and 26 will be supplied into the ANDcircuit 27 with a time difference of (30+τ) nanoseconds, where τ is thetime difference determined by the sweep rate and the distance betweenapertures 17' and 18'.

As is apparent from the above description, the apparatus according tothe sixth embodiment of the present invention offers the advantage thatit also has a capability of measuring temporal correlation for a longertime difference.

In the fifth and sixth embodiments as described above, electrons areshifted horizontally by means of the deflector 16. However, it should benoted that the same result can be attained by provision of a mirror 35as shown in FIG. 13, which is shifted in the direction perpendicular toan axial line A--A, that is, in the direction perpendicular to thesurface of the drawing, alternatively is rotated about the axial lineA--A. In a case where the mirror 35 is rotated, an fθ. lens should beprovided between the mirror 35 and the member 11. If no fθ lens isprovided, the amount of the incident fundamental particles to theaperture 10 of the member 11 is varied in accordance with a rotationangle of the mirror 35, and this is not preferable for measurement. Theprovision of the fθ lens can prevent the variation in the amount of thefundamental particles with the rotation angle of the mirror 35.

In the embodiments as described above, the member 11 having aperture 10is disposed in front of the fundamental particle-to-electron convertingsurface 12. If desired, an aperture may be provided in the acceleratingelectrode 13; this offers the advantage of eliminating the member 11without causing a decrease in efficiency on account of the open ratio ofthe accelerating electrode.

With reference to FIG. 9, the dynode groups 20 and 21 serving asmultiplication means are accommodated in a single tube together with thedeflectors 15 and 16 and the aperture member 19'. Alternatively, thearrangement as shown in FIG. 14 may be applied, in which a phosphorscreen 36 is disposed behind an aperture member 19' and photomultipliertube 37 and 38 are disposed behind the phosphor screen 36 so that thephotomultiplier tubes 37 and 38 correspond to the apertures 17' and 18',respectively. In this arrangement, electrons that have passed throughthe two apertures 17' and 18' strike the phosphor screen 36, and photonsgenerated from the phosphor screen are supplied into photomultipliertubes 37 and 38 and outputted as output signals to the correlators 22and 30.

In FIG. 10, the aperture 17' is designed in a slit form which iselongated in a horizontal direction, but this is not necessarily thecase. The aperture may be elongated at a certain angle with respect tothe horizontal direction.

The foregoing description assumes that electrons are swept in bothhorizontal and vertical directions, but this is not necessarily thecase. Electrons may be swept in other directions so long as they differfrom each other.

If temporal correlation of X-rays is to be determined, the fundamentalparticle-to-electron converting surface 12 is preferably made of Au orCsI, and for determining temporal correlation of neutrons, the surface12 is preferably made of U₂ O₃, U₃ O₈, or the like.

In all of the embodiments as described above, incident fundamentalparticles are converted to their respective images by the surface 12 andthe resulting images (electrons) are swept by the deflector 15 or thedeflectors 15 and 16. Alternatively, an electrooptical crystal may beemployed in such a manner that a change in its refractive index inresponse to an applied voltage is utilized to perform a direct sweepingof photons. The deflectors 15 and 16 may be designed to generate amagnetic field instead of an electric field.

Further, in all of the embodiments as described above, the member 11having aperture 10 is disposed in front of the fundamentalparticle-to-electron converting surface 12. If desired, an aperature maybe provided in the accelerating electrode 13; this offers the advantageof eliminating the member 11 without causing a decrease in efficiency onaccount of the open ratio of the accelerating electrode.

As described above, the apparatus of the present invention is sodesigned that fundamental particles or images thereof which have beenswept are time-divisionally extracted with two or more apertures, andarithmetic operations are performed on the extracted fundamentalparticles or images thereof for obtaining their temporal correlation.This design offers the advantage that temporal correlation for a veryshort time difference of the order of subpicoseconds can be obtainedwith high temporal resolution. A further advantage of the presentinvention is that such a subpicosecond time-resolved correlationmeasurement can be accomplished using a simplified processing system. Inaddition, the present invention enables temporal correlations to bemeasured successively while varying time differences.

What is claimed is:
 1. An apparatus for measuring the temporalcorrelation of fundamental particles comprising:Sweep means for sweepingthe fundamental particles or images thereof, extracting means fortime-divisionally extracting the swept fundamental particles or theimages thereof, multiplication means for multiplying each of theextracted fundamental particles or the images thereof, and Correlationmeans for performing correlating arithmetic operations on the basis ofeach output signal from said multiplication means.
 2. An apparatus asclaimed in claim 1, wherein said extracting means comprises an aperturemember having at least two apertures, the fundamental particles orimages thereof being time-divisionally extracted through said apertures.3. An apparatus as claimed in claim 1, wherein said multiplication meanscomprises dynode groups for respectively producing multiplied outputsignals from the image of said fundamental particles time-divisionallyextracted through said extracting means.
 4. An apparatus as claimed inclaim 1, wherein said multiplication means comprises photomultipliertubes for respectively producing multiplied output signals from thefundamental particles or images through time-divisionally extractedthrough said extracting means.
 5. An apparatus for measuring thetemporal correlation of fundamental particles comprising:fundamentalparticle-to-electron converting means for converting fundamentalparticles to electrons, accelerating means for accelerating theelectrons, focusing means for focusing the accelerated electrons,deflecting means for sweeping the electrons in at least one direction,extracting means having at least two apertures for extracting the sweptelectrons, multiplication means for multiplying the electrons extractedthrough said extracting means, and correlation means for performingcorrelating arithmetic operations on the basis of each output signalfrom said multiplication means.
 6. An apparatus as claimed in claim 5,wherein said multiplication means comprises at least two dynode groupsfor respectively multiplying the electrons extracted through saidextracting means.
 7. An apparatus as claimed in claim 5, wherein saidmultiplication means comprises a phosphor screen for converting electronimages extracted from said apertures of said extracting means to thecorresponding light images, and at least two photomultipliers forrespectively producing multiplied output signals corresponding to thelight images.
 8. An apparatus for measuring the temporal correlation offundamental particles comprising:fundamental particle-to-electronconverting means for converting fundamental particles to electrons;accelerating means for accelerating the electrons; focusing means forfocusing the accelerated electrons; deflecting means for sweeping theelectrons in at least one direction; extracting means having twoapertures for extracting the swept electrons, the apertures beingelongated along respective axes and relatively oriented to provide avariable distance between said axes; multiplication means formultiplying the electrons extracted through the extracting means; andcorrelation means for performing correlating arithmetic operations onthe basis of each output signal from the multiplication means.
 9. Anapparatus as claimed in claim 8, wherein the multiplication meansincludestwo dynode groups for respectively multiplying the electronsextracted through the extracting means.
 10. An apparatus as claimed inclaim 8, wherein the multiplication means includesa phosphor screen forconverting electron images extracted from the apertures of theextracting means to corresponding light images; and two photomultipliersfor respectively producing multiplied output signals corresponding tothe light images.
 11. An apparatus for measuring the temporalcorrelation of fundamental particles comprising:fundamentalparticle-to-electron converting means for converting fundamentalparticles to electrons; accelerating means for accelerating theelectrons; focusing means for focusing the accelerated electrons;deflecting means for sweeping the electrons in at least one direction;extracting means having two apertures for extracting the sweptelectrons, the apertures being elongated along axes paralleled with eachother; multiplication means for multiplying the electrons extractedthrough the extracting means; and correlation means for performingcorrelating arithmetic operations on the basis of each output signalfrom the multiplication means.
 12. An apparatus as claimed in claim 11,wherein the multiplication means includestwo dynode groups forrespectively multiplying the electrons extracted through the extractingmeans.
 13. An apparatus as claimed in claim 11, wherein themultiplication means includesa phosphor screen for converting electronimages extracted from the apertures of the extracting means tocorresponding light images; and two photomultipliers for respectivelyproducing multiplied output signals corresponding to the light images.